The present invention relates to a method of checking the irradiation control unit of an ion beam therapy system which, in particular, is operated with heavy ions.
Ion beam therapy systems are preferably used for the treatment of tumors. They have the advantage that when a target is irradiated, the major part of the energy of the ion beam is transferred to the target, while only a small amount of energy is transmitted to sound tissue. Therefore, a relatively high irradiation dose can be used to treat a patient. By contrast, X-rays transmit their energy to the same extent to the target and to sound tissue, so that, for health reasons in order to protect the patient, a high irradiation dose cannot be used.
U.S. Pat. No. 4,870,287, for example, discloses an ion beam therapy system in which proton beams are generated by a proton source, it being possible for its protons to be fed to various treatment or irradiation stations via an accelerator device. At each treatment station there is a rotating frame with a patient couch, so that the patient can be irradiated with the proton beam at different irradiation angles. While the patient is located physically at a fixed point within the rotating frame, the rotating frame rotates about the body of the patient in order to focus the irradiation beams at different irradiation angles onto the target, located at the isocenter of the rotating frame. The accelerator device comprises the combination of a linear accelerator (LINAC) and a synchrotron ring, as it is known.
In H. F. Weehuizen et al, CLOSED LOOP CONTROL OF A CYCLOTRON BEAM FOR PROTON THERAPY, KEK Proceedings 97-17, January 1998, a method of stabilizing the proton beam in proton beam therapy systems is proposed, the treatment beam being controlled actively in such a way that, at two measurement points spaced apart from each other in the longitudinal direction, it lies on the center line of the corresponding beam feed system. The first measurement point is located between a pair of deflection magnets and is formed by a multiwire ionization chamber. Depending on the current value, supplied by this multiwire ionization chamber, of the beam position with respect to the center of the beam path, the PI control of further deflection magnets, which are arranged upstream of the first-named pair of deflection magnets, is produced. The second measurement point is located shortly upstream of the isocenter and is formed by an ionization chamber subdivided into four quadrants. Depending on the current position value from this ionization chamber, again PI control signals are generated, but these are intended for the first-named deflection magnets. The intention of this control is to permit both angular stability with respect to the center line of the beam feed system and lateral positional stability of the proton beam.
When carrying out heavy ion irradiation, that is to say an irradiation using ions which are heavier than protons, large and heavy equipment is required, however, so that here there is the tendency to avoid the use of rotary frameworks and, instead, to move the patient or the patient couch. Corresponding therapy systems are described, for example, in E. Pedroni: Beam Delivery, Proc. 1st Int. Symposium on Hadrontherapy, Como, Italy, Oct. 18-21, 1993, page 434. These systems are accordingly eccentric systems.
Since, however, fundamentally isocentric systems are preferred by oncologists, a heavy ion beam therapy system has been proposed in which, although rotary frameworks are used at the treatment stations, the radii of the rotary frameworks can be reduced by the treatment beam fed to each rotary framework horizontally along its axis of rotation being guided, with the aid of suitable magnetic and optical arrangements, in such a way that it firstly runs away from the axis of rotation and subsequently crosses the axis of rotation again at the isocenter in order to irradiate a target. In order to irradiate the target, a raster scanner is provided, which comprises vertical deflection means and horizontal deflection means which each deflect the treatment beams at right angles to the beam axis, so that an area surrounding the target is scanned by the treatment beams. This system therefore substantially provides beam guidance in only one plane of the rotary framework.
The irradiation by means of the raster scanner is carried out with the aid of irradiation dose data, which are calculated by the control system of the ion beam therapy system automatically, depending on the patient to be irradiated or to be treated.
Since, in principle, high operational safety and operational stability with regard to the treatment beam are required of ion beam therapy systems, in the case of the heavy ion beam therapy system described previously, a monitoring device is provided to monitor the treatment beam supplied by the raster scanner. This monitoring device is arranged between the last deflection magnet of the aforementioned magnet arrangement and the isocenter, and may comprise ionization chambers for monitoring the particle flux and multiwire chambers for monitoring the beam position and the beam width.
During the operation of medical electron accelerators, various DIN standards have to be complied with for reasons of safety. These relate firstly to acceptance testing, that is to say checking the operational readiness, and secondly testing the constancy, that is to say checking the operational stability, of the system. For ion beam therapy systems, in particular for heavy ion beam therapy systems, such safety standards developed specifically for ion beam therapy systems are not yet known. However, in the case of ion beam therapy systems there is also the requirement for the greatest possible operational safety and operational stability.
The present invention is therefore based on the object of proposing a method of checking the irradiation control unit of an ion beam therapy system, in order to improve the operational safety and operational stability, in particular as referred to checking the irradiation control unit. At the same time, the intention is for the method to be suitable in particular for use with heavy ions.
According to the present invention, this object is achieved by a method having the features of claim 1. The dependent claims in each case define preferred and advantageous embodiments of the present invention.
According to the present invention, an ion beam therapy system is operated which comprises a raster scanner device arranged in a beam guidance system and having vertical deflection means and horizontal deflection means for the vertical and horizontal deflection of a treatment beam at right angles to its beam direction, so that the treatment beam is deflected by the raster scanner device onto an isocenter of the irradiation station and scans a specific area surrounding the isocenter, data sets and permanently stored data of a control computer, parameters of measuring sensors and desired current values of scanner magnets being checked. To this end, the data sets and programs to be loaded into front end processors from the central control computer are read back after being loaded and compared with the output data. The permanent storage of permanently stored data is checked. Desired current values of scanner magnet currents are compared with actual magnet current values. Calibration tests of measuring sensors are carried out.
The electrical charge produced in the ionization chambers of the monitoring system of the therapy system, said charge being used to determine the number of particles, depends on the pressure and the temperature of the ionization chamber gas, so that these two variables must be monitored and logged during the irradiation. The pressure and the temperature of the gas of the ionization chambers are measured with the aid of electrical sensors, the measured values being recorded about once per minute by the monitoring system, being converted into absolute units (hPa and xc2x0 C.) with input calibration factors and displayed digitally. The time variation of the measured values can be displayed graphically in a trend graph. The sensors are calibrated with the aid of reference measuring instruments. The calibration of the sensors fitted in the ionization chambers should be repeated before each therapy irradiation block. In addition, the air pressure and the room temperature at the location of the monitoring system are measured with absolutely calibrated instruments and registered by the monitoring system and also logged during each irradiation. Thus, for the (daily) checking of the ionziation chambers, the absolute values for air pressure and room temperature can be read off directly on the reference measuring instruments, compared with the values displayed by the monitoring system and logged. In this case, the reference values used are the measured values registered during the daily calibration of the monitoring system. In the event of a deviation of 28 hPa or 5xc2x0 C., respectively, an alarm is triggered by the monitoring system.
In addition, the loading of programs and data sets into the control computer of the ion beam therapy system must be checked. This is necessary in order to be able to load data which are required for a patient irradiation correctly into the sequence control of the system. Only if all the data are correct may a patient irradiation be started. For this purpose, with the aid of specific programs in the server computers of the monitoring system, programs and data are written into the individual processors of the control computers, are read back and compared with the programs and data stored in the dedicated memories, these test programs being executed automatically before each irradiation. Only if the data loaded back correspond exactly to the data stored in the data memories of the monitoring system can safe operation be assumed. In the event of deviations, an alarm message is generated, and the interlock unit previously described, which is used to prevent irradiation, cannot be released.
A further testing aspect relates to the switching of the currents for the deflection magnets of the raster scanner. In this case, it is necessary to ensure that the current values of these deflection magnets reach a determined desired value, set in the magnetic network devices, both in terms of value and also in terms of time within specific tolerance limits. For this purpose, the time between the setting of a magnet current value in the magnetic network devices and the reaching of the corresponding stable magnet current is measured for various current values. The maximum tolerable current accuracy with respect to a deviation from the set magnet current value is 0.3 A. The maximum tolerable setting time in the event of a current step of 2 A is 175 xcexcs in the x direction and 325 xcexcs in the y direction. If these tolerances are not complied with, the irradiation must be shut down. In order to check consistency, this test can be carried out before each irradiation block.
Finally, it is also necessary to ensure that the number of the irradiation point that is active when a shutdown condition occurs is stored permanently, that is to say secured against power failure. This permits the continuation of the irradiation, approved by authorized persons, at a later time. The serviceability of this implemented safety function can be checked by a specific irradiation or treatment plan being loaded into the monitoring system and executed without any irradiation, that is to say simulated. In the case of a specific irradiation site, the power supply of the sequence controller is switched off and, after the system has been restarted, the last irradiation site is read out and compared with the irradiation site when the power supply was switched off. If there is non-agreement, an appropriate intervention is made. To test consistency, this check is carried out before each irradiation block.
In particular, it is proposed to check the calculated irradiation dose values for a number of measurement points on the phantom, conclusions being drawn as to the adequate accuracy of the calculation of the irradiation dose data if the mean deviation between the calculated and measured values of the irradiation dose for all the measurement points does not exceed a predefined first tolerance value and, for each individual measurement point, the deviation between the calculated and measured irradiation dose for this measurement point does not exceed a predefined second tolerance. In this case, the first tolerance value is xc2x15% and the second tolerance value is xc2x17%.
In order to check the correct transmission of the geometric structures at the treatment station and the planning parameters of an image-providing device belonging to the ion beam therapy system as far as the positioning means, a digital reconstruction of the phantom, in particular a radiographic reconstruction, can be calculated and is compared with a radiograph of the phantom which is produced, in order to detect any possible deviation.
The present invention permits a considerable improvement in the operational stability and operational safety of the ion beam therapy system and defines a testing plan with specific testing aspects, which can be carried out with the effect of acceptance testing and/or consistency testing of the ion beam therapy system. This relates in particular to the irradiation planning, in the course of which irradiation dose data in the ion beam therapy system are calculated automatically, depending on the patient to be irradiated or treated.